I agree with Bork and Miller and believe simply that the ratifiers were trying to put former slaves on equal footing with all U.S. citizens. Just guessing (something Bork, admirably, was loathe to do which is why he might have been one of our greatest Justices if he had been given the chance), my sense is the ratifiers were simply wanting to confirm that states could not discriminate against black out-of staters. In that sense, P&I in the 14A doesn't add anything, only confirms the full citizenship status of former slaves. But that's my opinion only, not a constitutional basis for construction. I sense it's probably Bork's also, but he was disciplined and discrete enough to keep his personal opinions separate from valid constitutional construction.
If Privileges and Immunities are taken to mean ancient fundamental rights of Englishmen, then the meaning of the Clause is simple. No state may violate these fundamental rights.